Bubble Dynamics In Three Phase Fluidized Beds

An optical fiber probe was developed successfully for the study of bubble characteristics in three-phase fluidized beds. A single core optical fiber of 400 (mu)m was used in this study. The design of the probe was based on the difference of refraction indices between the gas and the liquid phases. The radius of the "U" bend was approximately 0.5 mm in this study.;The three phases used in the present study were air, water, and narrow cut glass beads of 250 (mu)m average diameter. The solid particles were fluidized by cocurrent upward flows of air and water with the later one as continuous phase. The experiments were performed in two different columns. The first one was a two dimensional bed which had a width of 65 cm and 2.5 cm in thickness. The other fluidized bed was a cylindrical column of 20 cm in diameter.;Bubbles in freely bubbling fluidized beds rose faster than single bubbles in liquids or in liquid fluidized beds. Log-normal bubble size distributions were found for most of the operating conditions studied. The distributions were widened by increasing the gas velocity.;Higher bubble frequency or gas hold-up was favored by increasing either the gas or the liquid velocity. The radial profiles of bubble frequency or gas hold-up were flat across the bed at low liquid velocity. Bell shaped profiles were observed when higher liquid flow rates were used.;In the axial direction, bubble frequency or gas hold-up increased slightly with height and this tendency was more noticeable at high liquid velocity. The solid and the liquid hold-ups were fairly constant in the lower two thirds of the fluidized bed. In the top region of the bed, the solid hold-up decreased rapidly and the liquid hold-up increased accordingly.;The average bubble velocity was obtained through the use of cross correlation and phase angle analysis. Material balances on gas phase were performed with the information of local gas hold-up and average bubble velocity. It was demonstrated that the consistency of measurements by the optical probes was satisfactory.